Low Drag Car Aerodynamics

Slipping easily through air

As every car
passed, you would be able to see the swirls and whirls of air disturbed by its
passage. Some cars would drag behind them an enormous wake, larger even than the
frontal area of the car. Others would have only a small area of disturbed air
trailing them, these cars slipping easily through the air.

If air could
be seen, you can be certain that car aerodynamics would have never gone out of
fashion – after all, boats with flat fronts are pretty rare!

Cd and
Frontal Area

There are two
factors that decide how easily a car can pass through air.

The most
commonly quoted factor is Cd, or coefficient of drag. A flat dinner plate moved
face-first through air has a Cd of about 1.1. More slippery shapes have a lower
Cd, such as the 0.45 Cd of a sphere.

The other
important factor is size. The frontal cross-sectional area of a car is the
height multiplied by the width, excluding open areas such as the space between
the wheels and including additional areas such as those of the rear vision
mirrors. (To gain an approximation of the frontal area, simply multiply height
by width.)

Cross-sectional area depends on how big the car is, but the Cd is
influenced by how the air flows over the car.

The total drag for a given speed is proportional
to Cd multiplied by the frontal cross-section, a figure termed CdA. Note that
the larger the car, the easier it is for its designers to achieve a low Cd – but
the CdA figure is the one that is more important. Incidentally, many
manufacturers’ Cd figures are quite rubbery - often, cars are retrospectively
given a poorer Cd figure after the next model is released.

How Air
Flows

If you
imagine air being a series of thin layers, when the airflow remains in layers
(laminar) as it passes over the car, the drag acting on the body is low.
Anything that causes the laminar flow of air to separate from the body - and so
become turbulent - causes drag. The ultimate low-drag shape is a teardrop with
its hemispherical front end and long, tapering tail. Theoretically, a tear drop
shape has a Cd of only 0.05! Note that it’s not the pointy end that faces
forward, but the rounded end which goes first.

It’s worth
thinking about why a tear-drop shape has such a low drag co-efficient.

When the
smoothly-curved front end of the teardrop shape meets the air, the air is gently
deflected around it, staying attached to the object in attached flow. The long
tail of the drop allows the flows to rejoin with only very minor turbulence.

If the air
had not remained attached to the shape (because of a sudden step in the shape,
for example), the flow would have become turbulent at that point.

However, it is obviously impractical to have a car
shaped like a teardrop. The closest that road vehicles currently come to this
are the solar race cars – vehicles that have incredibly low Cd values (eg 0.1)
matched with very low cross-sectional areas. These cars can travel at 100 km/h
using only 1.5-2kW (2-2.7hp) of power. Even in normal road cars, the basic rules
of having smooth surfaces with no abrupt transitions of shape, gentle front
curves and long tapering tails continue to apply.

When a car moves, air is deflected above, below
and around it. The point at which the air splits to pass above or below the car
(termed the stagnation point) is important in deciding how slippery the car will
be. The lower the stagnation point, the better, because then less air runs into
the (usually) rough underside of the car. However, unless the car has a front
spoiler extending almost to the ground, air will pass under the car. This has caused some
manufacturers to start adopting low drag undersides for their cars. There is
also another reason for entraining this air into a smooth flow – creating less
lift, which we’ll get to in a moment.

So if the bulk of air doesn’t pass under the car,
where does it go? Combustion air is drawn into the engine, usually from near the
front of the car. This air movement normally doesn’t create much drag but the
flows of cooling air through the radiator do create a lot of drag. On a car with a front
radiator, a huge amount of air enters through the cooling duct opening, is
forced to flow through the radiator, and then spills out untidily underneath the
car. This turbulent movement of the radiator cooling air increases the Cd figure
by as much as 10 per cent. For example, the radiator cooling airflow accounts
for 8 per cent of the AU Ford Falcon’s drag. This means that without this drag
penalty, the car’s Cd would drop from 0.295 to 0.271! (Back in the days of the
AU Falcon, manufacturers actually released data on their cars’
aero-effectiveness.)

The use of a
radiator intake duct that retains attached airflow for at least most of its
length will give the best flow with the least drag. Controlling the flow of air
after it leaves the radiator core is also important. Ducting air out through the
wheel wells is efficient, but even better is the ducting of the air out through
the top of the bonnet. Many sports homologation versions of road cars have taken
the approach, fitting special vents to flow radiator air out through the bonnet.
However, care needs to be taken that this flow does not disrupt the attached
flow across the upper surface of the bonnet.

Air which is
forced over the top of the car has to make its way from the relatively bluff
nose area to the top of the bonnet, staying attached over the transition formed
by the leading edge of the bonnet. This radius is critical. If this corner is
too sharp, the air will separate from the car’s surface. A separation bubble
will form on the bonnet, leading to the presence of turbulence at the front of
the car. It is for this reason that most modern cars have very gentle
transitions of shape in this area. One of the all-time great aero specials – the
1970 Plymouth Roadrunner Superbird with its enormous rear wing – had a curved
nosecone substituted for its normally bluff front. This extension prevented
bonnet separation and also reduced the amount of air passing underneath the
car.

The radius
formed by the transition from windscreen to roof is also vital. If this angle is
too sharp, the airflow may not remain attached to the roof, causing turbulence
and further problems towards the back of the car - it’s not much good having a
rear wing placed in turbulent air!

What happens at the back of the car is
extremely important in determining total drag, rear axle
lift and, to a more limited extent, front axle lift. In many cases, the flow at
the back of the car is more important than the flow behaviour at the front. The
pattern of airflow at the rear of the car depends very much on the type of car
being examined. If the airflow from the roof is to remain attached down onto the
boot, a three-box sedan must have a very shallow-angled rear window. It is very
important that this flow does remain attached – the area of the
wake will be reduced, dramatically lowering the car’s overall Cd. When the
profile of a three-box sedan is compared with an ultimate teardrop shape, it can
be seen that separation at the roof/rear window transition can easily
occur.

Cars with
gently sloping rear windows – often hatchbacks or coupes – allow the airstream
to remain attached right to the rear of the car, so producing only a small wake.
The transitional curve between the roof and the hatch needs to be gentle if the
airflow is to remain attached, and the angle of the hatch to the horizontal is
also critical. It’s important to note that while it may look ‘obvious’ to the
eye that the airflow remains attached across a coupe or hatchback’s rear, wool
tuft testing needs to be carried out to prove this.

Even quite minor changes in rear hatch angle can
cause major changes in drag. Tests carried out by Volkswagen have shown that the
Cd of the car can vary from 0.34 to 0.44 as a result of slight alterations to
the rear hatch angle. At one angle (30 degrees to the horizontal in this case)
the airflow separation point jumped back and forth from the end of the roof to
the bottom of the hatch, depending upon the curvature at the rear edge of the
roof. It was this 30 degree rear hatch angle that produced the highest Cd value.
This sort of substantial change in the car’s drag coefficient will have a large
influence on the car’s top speed and fuel consumption. In some cars even a 10
per cent reduction in drag will decrease open road fuel consumption by 5 per
cent.

Cars with a
near-vertical rear hatch have airflow separation that occurs at the end of the
roof. This means that the wake is as large as the frontal cross sectional area
of the car. People who locate spoilers half way down the rear hatch should
realise that they are achieving nothing with this placement!

A large wake
is also present on most station wagon designs. For example, the Cd of the AU
Ford Falcon wagon is sedan is 0.341, versus the sedan’s 0.295.

Side
Flow

Airflow along
the sides of the car is also important. The use of flush-mounted side glass is
one approach that is taken to reduce the surface roughness; however, this is
implemented more for noise reduction than for lowering drag. Rear vision mirrors
have also changed in shape as their aerodynamic drag and the way in which they
influence the behaviour of the airflow further down the side of the car is taken
into account.

Also very
important is the plan view shape of the A and C pillars. Very curved pillars are
used to encourage the flow of air from the windscreen smoothly around onto the
side glass. On three box sedans, the greater the flow of air that can be gained
from the sides of the car onto the boot lid, the better. This air can be used
with aerodynamic aids such as spoilers, and its presence also helps fill that
‘hole’ in the air created by the car’s forward movement, thus lowering drag.

A classic
case of modification of the shape of a car to achieve laminar flow down its
sides occurred way back with the design of the very first Volkswagen Kombi.
Initially, the vehicle had an almost flat front and very sharp corners – a bit
like a moving shoebox! In this form, severe turbulence occurred down each side
of the vehicle and it had a Cd of 0.76. Slight rounding of the nose was then
carried out, with special attention paid to smoothing the transition from the
front to the sides of the vehicle. The Cd then dropped to just 0.42!

Wakes

As indicated previously, a car draws along a wake
of disturbed air. The smaller this disturbance, the lower will be the drag. A
very long tail which tapers in both height and width will cause the least drag –
that teardrop shape again! While long, tapered tails aren’t practical in normal
car design, a truncated version is often used. Cars using boat-tailing (a
narrowing of the width of the rear when looked at in plan view) are common, with
measured drag reductions of up to 13 percent achieved with this approach. The
Opel Calibra – still one of the all-time aero greats - decreased its Cd by 0.01
with a total rear boat-tailing of 130mm.

Cars with
tails that taper in profile are also common, but often the taper is abruptly cut
off. This is a called a Kamm tail.

The flow of
air over the car will create high and low pressure areas. Low pressure areas
(creating lift and sometimes drag) occur most frequently where the airstream
passes over an upper curved surface – the transition from grille to bonnet,
windscreen to roof, and roof to rear window. The shape of the original Porsche
911 created very high lift coefficients - it’s a good example of where laminar
airflow wraps around a long, upper-body curve. High pressure areas (creating
downforce and sometimes drag) occur at the very front of the car, and in the
transition from bonnet to the base of the windscreen.

The way in
which these pressures act will result in an overall lift coefficient, or Cl. The
lift coefficient is normally expressed for both the front axle (Clf) and rear
axle (Clr). A negative lift value (shown by a minus sign in front of the
coefficient) indicates that downforce occurs – something relatively rare in a
road car.

The cabin
ventilation inlets and outlets provide good clues as to the location of
(respectively) high and low pressure areas. Intakes for the cabin ventilation
system are almost always at the base of the windscreen, while outlet vents are
placed in a range of areas. On most recent cars, the outlets are hidden behind
the bumper bar in the low pressure wake, but in older cars, vents in the
C-pillar or across the top of the rear window are used. While the location of
these vents is not of direct use in modifying cars, looking at cars and thinking
about the pressures present at these vent locations can be quite
illuminating.

Conclusion

The current automotive fashion is for
manufacturers to pay only lip service to aerodynamics. In fact, there has been
little improvement in CdA figures in the last decade. However, at highway
speeds, most of the power being developed by the engine is being used to push
air out of the way. For cars of the future to improve their open road fuel
consumption, CdA figures will have to once again fall: it is simply a physical
requirement of efficient car design.